Quantitative determination of bone mineral mass is used for diagnosis of osteoporosis and the like.
Quantitative determination of bone mineral mass in bone marrow as an objective region will be outlined hereinbelow.
As shown in FIG. 8, phantom P is placed below the waist of a subject. Then, scanning with a CT scanner yields the image data of the cross section including the bone marrow (for example, the third lumbar vertebrae) and the phantom P. Herein, the phantom P contains plural sample rods S1, S2, . . . , produced by mixing a water equivalent material having the same X-ray transmission rate as that of water with various ratios of a standard material equivalent to bone mineral mass which has the same X-ray transmission rate as that of bone mineral mass.
Then, detecting CT numbers A1, A2, . . . of the sample rods S1, S2, . . . in the cross sectional image data, a linear regression "e" representing the relation between the CT number and the bone mineral density of the standard material equivalent to bone mineral mass as shown in FIG. 9 is calculated on the basis of the CT numbers A1, A2, . . . and the densities of the standard material equivalent to bone mineral mass in the sample rods S1, S2, . . . The X axis represents bone mineral density, while the y axis represents CT number.
Then, detecting the CT number of the bone marrow as the objective region in the cross sectional image date, the bone mineral density of the bone marrow is calculated on the basis of the detected CT number and the linear regression "e".
As illustrated in FIG. 10, however, the bone mineral density calculated on the basis of the linear regression "e" is sometimes inconsistent with the true value.
For example, a linear regression generated from scanning at an X-ray tube voltage of 80 kV is designated "e80"; the CT number of bone marrow is designated "Aq80"; and the bone mineral density derived from these is designated "x80". Alternatively, a linear regression generated from scanning at an X-ray tube voltage of 140 kV is designated "e140"; the CT number of bone marrow is designated "Aq140"; and the bone mineral density derived from these is designated "x140". Then, the bone mineral density "xt", which should be constant irrespective to the difference in tube voltage, is actually not constant as illustrated in x80&lt;x140. Such results may possibly be due to some error factors. Therefore, since the decrease in CT number due to fat may be one of such error factors, the shift of measured bone mineral density from the true bone mineral density "xt" at each tube voltage is calculated while the decrease in CT number due to fat is designated "af". Then, x80 should be more than x140, as shown in FIG. 11, which is not consistent with the above results shown in FIG. 10. Thus, such calculation of bone mineral density as shown in FIG. 10 may possibly contain a certain error factor other than fat, but the factor has not been identified yet. Hence, the error factor due to fat has not been excluded either.